ICON Ford Raptor Hydraulic Bumpstop Testing + Tuning

loki_13 said:

Although they are hydraulic they still have nitrogen in them. That little valve on the top is where you charge it to the proper psi. It has to do with valving or psi. my question is, what happens when the bump hasn't retracted all the way and you hit another bump. Do you just slam metal to metal?

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Good question Paul.

loki_13 said:

Although they are hydraulic they still have nitrogen in them. That little valve on the top is where you charge it to the proper psi. It has to do with valving or psi. my question is, what happens when the bump hasn't retracted all the way and you hit another bump. Do you just slam metal to metal?

Click to expand...

The bumpstops are hydraulic and nitrogen (air). I know the common nomenclature is misleading. Technically they are hydro-pneumatic bump shocks but if we refer to them as such then they would never pop up on the Google searches.

The rebound rate is the hardest and most important part of the bumpstop tuning. If we knew that the bump occurrences were far apart we would tune the rebound to be very slow so that almost none of the impact energy would be returned to the system. The characteristic is that after impact the axle “disconnects” from the bumpstop and rebounds on the “connected” spring and shock. The other factor is that because the “spring” function of the bumpstop comes from compression of the nitrogen inside, the rate is very non linear. This is great for bottom out resistance but difficult for rebound tuning. We combat this to some extent by using a piston that is digressive on rebound. The bumpstop will rebound much faster in the first half of the extension stroke than the second however. If there are two closely spaced successive hits it is possible for the bumpstop to only rebound partially. When this happens the second hit usually has less energy because it’s not coming in with as much velocity as it would if the suspension had extended further so it does not require as much to control. If it had extended further the bumpstop would have fully extended also. Secondly the bump stop will have more residual pressure in its partially extended state and thus will have higher initial contact force to handle the situation.

When tuning a bumpstop application I do most of the nitrogen pressure, compression valving and oil fill volume determination with lots of field testing. The rebound tuneing is done with both video from the field testing and shock dyno work. On the dyno we can see how much force is being imparted to the suspension on the return stroke. We also do field data acquisition that plots the position and velocity of the suspension so we know what velocities correlate to what desired or undesirable reaction. Without the data acquisition the dyno speeds to analyze would just be a guess.

The goal is to get it to return as fast as possible but not kick back. I can usually get it to follow the suspension back out but not push it back out. The last 10-20% of the stroke is at very high pressure and usually pushes back a bit. The middle 60-70% will follow the suspension out with little force and the last 20% will disconnect and extend on its own.

The other important factor is how far the bumpstop is from contact at ride height and the percent of bumpstop travel compared to overall travel and up travel from ride height. With about 5” of total up travel and 2.5” of bumpstop travel the ratio is not what I would ideally want but convincing people that they need to stink bug their trucks even though it doesn’t look cool is not going to fly. I do have a 1.9” travel bump stop but there’s a lot of work to be done in not much room

These are very cool looking and well built! I can't wait to see on in person.

Great Explanation

I've got one of these kits on the way. I'll be sure to try to post some more pics of it and maybe do a write up on it after I get it installed.

Netix said:

time to get aftermarket fenders lol. did that rub on the pic?
cool video though!

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Nah fenders are good....we run 35's during heavy offroad and it works out perfect. I think Dylan did some minor clearance work on some plastic and what not, but nothing that is any big deal. The 37's would def smash the fenders with out some mods though.

DylanICON said:

The bumpstops are hydraulic and nitrogen (air). I know the common nomenclature is misleading. Technically they are hydro-pneumatic bump shocks but if we refer to them as such then they would never pop up on the Google searches.

The rebound rate is the hardest and most important part of the bumpstop tuning. If we knew that the bump occurrences were far apart we would tune the rebound to be very slow so that almost none of the impact energy would be returned to the system. The characteristic is that after impact the axle “disconnects” from the bumpstop and rebounds on the “connected” spring and shock. The other factor is that because the “spring” function of the bumpstop comes from compression of the nitrogen inside, the rate is very non linear. This is great for bottom out resistance but difficult for rebound tuning. We combat this to some extent by using a piston that is digressive on rebound. The bumpstop will rebound much faster in the first half of the extension stroke than the second however. If there are two closely spaced successive hits it is possible for the bumpstop to only rebound partially. When this happens the second hit usually has less energy because it’s not coming in with as much velocity as it would if the suspension had extended further so it does not require as much to control. If it had extended further the bumpstop would have fully extended also. Secondly the bump stop will have more residual pressure in its partially extended state and thus will have higher initial contact force to handle the situation.

When tuning a bumpstop application I do most of the nitrogen pressure, compression valving and oil fill volume determination with lots of field testing. The rebound tuneing is done with both video from the field testing and shock dyno work. On the dyno we can see how much force is being imparted to the suspension on the return stroke. We also do field data acquisition that plots the position and velocity of the suspension so we know what velocities correlate to what desired or undesirable reaction. Without the data acquisition the dyno speeds to analyze would just be a guess.

The goal is to get it to return as fast as possible but not kick back. I can usually get it to follow the suspension back out but not push it back out. The last 10-20% of the stroke is at very high pressure and usually pushes back a bit. The middle 60-70% will follow the suspension out with little force and the last 20% will disconnect and extend on its own.

The other important factor is how far the bumpstop is from contact at ride height and the percent of bumpstop travel compared to overall travel and up travel from ride height. With about 5” of total up travel and 2.5” of bumpstop travel the ratio is not what I would ideally want but convincing people that they need to stink bug their trucks even though it doesn’t look cool is not going to fly. I do have a 1.9” travel bump stop but there’s a lot of work to be done in not much room

Click to expand...

“We combat this to some extent by using a piston that is digressive on rebound.”
So you are using a piston with different flow characteristics for rebound and for compression. That makes sense.
“The bumpstop will rebound much faster in the first half of the extension stroke than the second however.”
How drastic could you change or slow movement with shims.? Unless it is internally bypassing the piston.
“If there are two closely spaced successive hits it is possible for the bumpstop to only rebound partially. When this happens the second hit usually has less energy because it’s not coming in with as much velocity as it would if the suspension had extended further so it does not require as much to control.”
Im not sure I agree with this statement. If 2 consecutive hits were to occur the bump stop, shock, and spring would not be fully collapsed thus needing less travel to achieve full bump . Correct?
“Secondly the bump stop will have more residual pressure in its partially extended state and thus will have higher initial contact force to handle the situation.”
How much residual pressure and enough to really make a difference in a 6000lbs truck driving through the 3ft. desert woops at 60mph. The last time I charged a shock at full extension to 200lbs and set it on the wheels the pressure grew to 210lbs so is that 10lbs of residual pressure going to make that difference on a second occurring impact to handle the situation? What is the growth in a icon 2.0 bumpstop at .5 and full compression?

Thank you for your response.

The compression ratio for a shock is kept as low as possible and usual way less than 2:1. The hydraulic bumpstop has a compression ratio from 8:1 for light apps and can go to theoretically infinity. For the raptor its 14:1. We can approximate the pressure to roughly double every time half the remaining travel is used for most of the travel. If you started at any point on the curve it will continue along that progression.
14:1 compression ratio
X FORCE(LBf) RATE(LB/IN) PRES.(PSI)
0 184 68 150
0.13 193 75 157
0.25 203 83 165
0.38 214 92 174
0.5 226 103 184
0.63 240 116 195
0.75 255 132 208
0.88 273 150 222
1 293 174 239
1.13 316 202 258
1.25 344 239 280
1.38 377 287 307
1.5 416 350 339
1.63 465 437 379
1.75 527 561 430
1.88 608 747 495
2 718 1042 585
2.13 878 1556 715
2.25 1127 2568 919
2.38 1576 5018 1284
2.5 2618 13846 2133

Almost infinite compression ratio
X FORCE(LBf) RATE(LB/IN) PRES.(PSI)
0 184 74 150
0.13 194 82 158
0.25 205 91 167
0.38 217 102 176
0.5 230 115 187
0.63 245 131 200
0.75 263 150 214
0.88 283 174 231
1 307 204 250
1.13 335 243 273
1.25 368 294 300
1.38 409 363 333
1.5 460 459 375
1.63 525 599 428
1.75 613 815 499
1.88 735 1173 599
2 918 1830 748
2.13 1223 3246 996
2.25 1830 7271 1491
2.38 3636 28707 2963
2.5 278213 168083361 226708



The reality of the situation is that the static pressure is only part of the story because the dynamic forces created depending on speed of impact can far exceed this simple pressure analysis. Double tap is a rare occurrence and most of them are light partial compression situations not big hits. Partial recovery from a full hit is not as marginal as implied, it is more in the 80% range so now it acts like a 2" travel bumpstop @180 psi and is still velocity sensitive and the pressure component that is significant is happening much later. Just like it would be a bad idea to tune a motor for just wide open throttle we need to tune every other component for a good balance of performance and to avoid the bad stuff first. If we tuned the bumpstop to absolutely fully extend in every odd ball situation it would "kick like a mule and bite like a crocodile".
We are also giving the bumpstop to much credit for its role in the vehicle dynamics as a whole. The reason it gets disproportional importance on the Raptor is because the frame is weak above the factory bumpstop making it a priority on the mod list to avoid damage, and there just simply isn't enough up travel in the rear.
Imagine the unlikely obstacle of a stair case that you have miraculously avoided with the front tires. The first stair would drive the suspension up and depending on the tread length the next stair would immediately drive it up again. If you do this at 60 I'm sorry but your screwed, at 15 no problem.
The beauty of the system is that they are tunable. You can increase just the nitrogen pressure and it will rebound faster. At some point you will notice that the kick tendency outweighs the gained compression force. And a corresponding rebound change is needed.